1. Field of the Invention
The present invention is in the field of silicone polymers. More particularly the present invention relates to optically clear, high temperature resistant silicone polymers which have a high refractive index. The present invention also relates to the process of making such polymers. Further, the present invention relates to the application of the polymers of the invention to the fields of optical waveguides, high brightness light emitting diodes (HBLEDs), photovoltaic devices, vertical cavity surface emitting lasers (VCSELs), laser diodes, light sensing devices, flat panel displays, projection display optics components, and injection-moldable optical lenses and other optical parts, devices and structures.
2. Brief Description of Background Art
Silicone polymers per se are well known in the art. U.S. Pat. No. 5,217,811 discloses a vinyl-terminated dimethyldiphenylsiloxane copolymer crosslinked with tri- or tetrafunctional silanes. The optical refractive index (nD) of this crosslinked silicone copolymer can be adjusted by varying the phenyl group content of the copolymer. The highest optical refractive index disclosed in this reference appears to be 1.50. The cross-linked silicone copolymers of this reference are used as index matching materials for connections between optical components, such as optical fibers.
A persistent difficulty in the field of optical polymer materials is the high attenuation of such materials in the near ultraviolet (UV), visible, and near-infrared (NIR) portions of the electromagnetic spectrum.
Optical clarity is expressed as the percentage of light transmission over a one-centimeter path length at the wavelength of interest. Equivalently, the complementary property of optical absorption is expressed as decibels of light signal loss per centimeter of optical path length (dB/cm). For adequate optical clarity in the field of optical waveguides used in the NIR, optical polymers should preferably exhibit absorption of less than 0.2 dB/cm. (See Advances in Polymer Integrated Optics, EEE Journal of Selected Topics in Quantum Electronics, Volume 6 No. 1, 2000 by Eldada et al.). In the field of molded plastic optical lenses, the lens thickness is typically in the range of 1 to 5 millimeters. For visible light applications such as white light emitting diode (LED) lenses, camera lenses, or implantable intraoccular lenses, it is desirable to maintain optical transmission over the visible range which is sufficient to prevent any detection of haze or color tint by the human eye. This degree of visually perceived clarity is often denoted “crystal clear”, or equivalently “water white”. For such devices, an optical polymer exhibiting water white clarity should exhibit absorption less than 0.2 dB/cm or equivalently should have a transmission of greater than 90%, preferably greater than 95%, in one centimeter across the visible range of wavelengths, and in particular at the short wavelength end of the visible spectrum.
In the fields of HBLEDs, photovoltaic cells, VCSELs, high efficiency photosensors, and flat panel displays, optical polymers may be used to form an encapsulating layer between the light emitting or light sensing element itself and an outer lens or cover glass, or may serve as both encapsulant and outer lens simultaneously. The degree of desired optical transmission in these applications again is typically greater than 90%, preferably greater than 95% over a one centimeter path at the wavelength of interest.
A further difficulty in the field of optical polymers is the requirement for such materials to withstand high temperatures without degradation in mechanical or optical properties. There are two high temperature conditions that are commonly encountered: steady state high temperature service, and transient high temperature service. Steady state high temperature service represents the maximum temperature to which the optical device is subjected for long continuous periods, of the order of hours up to many thousand of hours. Examples of steady state high temperature service range from approximately 30° C. to 40° C. for laboratory instrument optics and intraoccular implants, to 85° C. for telecommunications fiber optic control components, to 140° C. for automotive and aerospace optics. Transient high temperature service limits are usually due to exposure of the device to processing temperatures at later steps of manufacture with exposure times of the order of minutes. Commonly encountered high temperature processes range from lead alloy soldering (150° C. to 200° C.) to lead-free soldering (200° C. to 260° C.).
When exposed to high temperature service conditions, many optical polymers will degrade in performance. Mechanical degradation takes the form of hardening, outgassing of volatiles, embrittlement, crazing, cracking, shrinking, melting, or delamination of the polymer from substrates. Optical performance degradation can take the form of increases in optical absorption noticeable visually as a transition from water white to yellow or brown color tint, or a development of milkiness or haze. Quantitatively, this type of degradation usually is more extreme at the short wavelength end of the visible spectrum at 400 nm than at the long wavelength end of the spectrum at 750 nm. Yellowing typically becomes visually noticeable when the transmission at 400 nm falls below 90% and is usually considered unacceptable when it falls below 80% at 400 nm. It is typically the case for optical polymers that temperature-induced increases in absorption occur first at the shortest wavelengths (e.g. 400 nm) and are not apparent at the longer visible or NIR wavelengths (e.g. 1300 nm, 1550 nm) until much more severe exposure has occurred. For this reason, acceptable optical transmission degradation of an optical polymer at 400 nm induced by high temperature service is normally a strong indicator that any such degradation will be either undetectable or certainly acceptable at longer visible and NIR wavelengths. In addition, some of the mechanical degradation symptoms listed above such as crazing or delamination can also create large optical signal reflection or absorption.
Yet another difficulty in application of polymers to many optical and related devices is the need for polymers to also have, in addition to optical clarity and high temperature service, the advantageous property of high refractive index. In the context of the present description, “high refractive index” is taken to mean an index of refraction greater than or equal to 1.545 at 589 nm. High refractive index is advantageous in many optics designs for improving the light transmission efficiency of the design, or for reducing the required size of the optical assembly. For example, in the field of HBLEDs, the light extraction efficiency of the LED die is increased when high refractive index die encapsulants are employed. Specifically, the publication ENCYCLOPEDIA OF CHEMICAL TECHNOLOGY fourth Edition, Volume 15 pp 225–226 (John Wiley & Sons Publishers) discloses certain operational characteristics of light emitting diodes and teaches that epoxy, utilized for encapsulation of the light-emitting diode, typically has an optical refractive index of approximately 1.6. The equation disclosed in this reference provides the mathematical explanation for the requirement mentioned above that for a light-emitting diode to function efficiently the refractive index of its encapsulating coating must be high, preferably over 1.545 even more preferably over 1.56.
In the field of intraoccular lenses, use of a high refractive index polymer for the lens material permits the same focusing power with a lens which is smaller in physical size.
In the field of planar optical waveguides, a high refractive index waveguide core improves the waveguide bend leakage, and permits shorter waveguide runs for phase delay elements.
Another persistent difficulty in the field of optical polymers is the capability of the polymer to withstand exposure to radiation of high intensity. Many of the devices employing optical polymers require such radiation exposure for extended periods of time. For example, polymers used in the construction of projection optics, HBLEDs, and VCSELs are routinely exposed to non-ionizing radiation which may cause localized heating of the polymer, thereby enhancing its degradation through the temperature service effects described above. Exposure to ionizing radiation also occurs in some devices. For example, flat panel displays used in outdoor applications and automotive taillight LEDs are exposed to the UV radiation of sunlight; HBLED dies operating below 400 nm emission wavelength transmit near UV radiation which passes through optical polymer encapsulants and lenses. The presence of intense ionizing radiation is known from the prior art to degrade many types of optical polymers. However, it is known from the prior art of polymers used in lubrication and plastic compositions for the nuclear industry that highly phenylated polymers exhibit improved resistance to radiation damage.
Still another persistent difficulty in the field of optical polymers is the need for materials that can be varied in elastic modulus across a range from very soft gel materials to hard plastic materials. Such variation in elastic properties is often desirable in certain optical devices for the purpose of reducing internal stresses during temperature cycling, and for matching dimensional shifts due to expansion and contraction of adjacent materials with different coefficients of thermal expansion. While some of the aforementioned polymer technologies of the prior art, notably some of the silicones, are capable of wide variations in elastic modulus, most are available only with the modulus values of hard plastics, measuring on the Shore D durometer scale of hardness of ASTM D-2240.
It is instructive to summarize the shortcomings of optical polymers heretofore employed in several optical and related devices where the silicone composition of the present invention can be advantageously employed. Thus, prior art polymers generally exhibit one or two of the desirable properties of optical clarity, high temperature service, and high refractive index, but do not exhibit all three properties simultaneously. For example, prior art polymers which may have adequate optical clarity and high temperature service but inadequately high refractive index include fluorosilicones, dimethyl silicones, some phenylmethyl silicones and amorphous perfluoropolymers, amorphous fluoroplastics, and other amorphous halogenated plastics. See for example U.S. Pat. No. 6,204,523 that discloses light-emitting diodes encapsulated with silicone compositions that are transparent in the green-to-near UV wavelength range, approximately 570 to 350 nm.
A second class of prior art polymers has adequately high refractive index and high temperature service capability but inadequate optical clarity; such polymers may include: polyetherimides, and polyimides.
A third class of prior art polymers has adequate optical clarity, and some of which have adequately high refractive index but which are incapable of high temperature service. These include optically clear epoxides, polymethylmethacrylates, acrylics, polyvinylchlorides, cyclic olefin copolymers, polyurethanes, cellulose acetate butyrates, polycarbonates, and polystyrenes. In contrast to all of the foregoing types of conventional polymers used for optical device applications, the phenyl silicone resin compositions of the present invention, when applied to the optical devices described above, simultaneously exhibit the requisite properties of optical clarity, high temperature service and high refractive index.
It is well known in the art that silicone polymers are generally speaking more resistant to heat than other polymers. Nevertheless, as far as the present inventors are aware, optically clear crosslinkable liquid silicone polymers have not been made to this date with an optical refractive index greater than 1.54. This is in spite of the fact that U.S. Pat. No. 6,204,523 states that silicone has a high refractive index (1.4 through 1.7). However, this reference patent does not disclose any specific silicone composition and provides no basis for the assertion that a silicone composition of such high refractive index has ever been made.
In light of the foregoing, there is a need in the art for a silicone composition which has an optical refractive index in the range of 1.545 to 1.60, has high thermal stability and resistance to aging (yellowing) and is transparent to light down to the wavelength of 400 nm. In addition there is a need in the prior art for compositions which have the above-noted properties and additionally have the improved capability of withstanding radiation of high intensity and the capability of being formulated with an elastic modulus that ranges from very soft materials to hard plastics. The present invention provides such compositions.